Introduction
          Take out a healthy garden plant. Wash its roots to remove soil particles sticking to them. Take its weight (say 2 kg). It is its fresh weight. Dry the plant in an open place and again take the weight (say it is 100 gms). It is its dry weight. Burn the dried plant. We get ash. Weight it (it may be 20 gms). The ash consists all the minerals of the plant in the form of oxides etc.
History:
          It was first Aristotle (384 - 324 B.C.) who suggested that humus (soil) is the food of plants. For 2000 yrs there was no opposition to it. Von Helmont proposed that water is the food of plants. Woodward (1699) worked a spearmint (Mentha spicata) and established that soil, along with water, is essential for growth of the plant. In 1804 de Saussure found that the increase in dry weight of the plants is due to the minerals dissolved in water (More of the dry weight of the plants is absorbed from soil). In 1850, Julius Von Sachs (German) & W. Knops did experiments by growing plants in dilute solution (without soil) and discovered some essential elements required for plants' growth. This technique of growing plants in specific nutrient dilute solution without the soil is called Hydroponics or soil less culture or solution culture. This term Hydroponics was coined by Gerick.

          The study of low plants obtain mineral elements from outside regularly and utilise them for growth, development and to complete life cycle is called Mineral nutrition. It is also called Inorganic nutrition because all the minerals are Inorganic in origin.
           Plants consist about 70 to 80 kinds of minerals in their body. But all these are not present in one plant. A plant on average may consist 30 - 40 elements. All these are not useful to them.
Types of Elements:
          These are divided into 3 types.
1. Essential elements: The elements whose deficiency cause decrease in growth or incomplete life cycle are called essential elements.
2. Non - essential elements: These are present in the plant body but not useful in any way.
3. Beneficial elements: These are not essential but their presence in the body may be useful in one way or other.
e.g.: Sodium, Selenium, Silicon and Cobalt.
Essential Elements: Criteria of Essentiality: Daniel Arnon and Perry Stout worked on Lycopersicon esculentum (Tomato) and proposed 3 criteria of essentiality.

They are
1. The requirement of the element must be specific and replaceable.
2. The element must participate directly in the metabolism.
3. In the absence of the element, the life cycle is incomplete. Basing on the above criteria 16 elements are formed to be essential. Very recently Nickel is also added to the list. Thus the total number of essential elements become 17.
Essential Elements are divided into 2 types on the basis of quantity of requirement.
1. Macro Elements or Major Elements: The elements which are present in the plant tissues in large amounts (more than 10 m mole/kg of dry matter) are called Macro Elements or Macro Nutrients. Not only quantity, they should also form constituents of protoplasmic substances. There are 9 macro elements.
Carbon, Hydrogen, Oxygen, Nitrogen, Calcium, Magnesium, Potassium, Phosphorous and Sulphur.
2. Micro Elements or Minor Elements or Trace elements or Micro nutrients: The elements which are present in the plant tissues in very small amounts (less than 10 m mole/kg of dry matter) are called micro elements. There are 8 micro elements. They are Fe, Cu, Zn, Mn, Mo, B, Cl and Ni.

Origin of the essential elements:
    Carbon, Hydrogen and Oxygen are absorbed from atmosphere in the form of CO2 (atmosphere) and H2O (Soil). These are considered as Non - mineral elements. Nitrogen is regarded as mineral as well as non - mineral element. Rest of the elements (13) are considered as mineral elements.
Essential elements are divided into 4 categories basing on their functions.
1. Framework elements: C, H and O are considered as framework elements because they are found in the entire body of the plant body.
Dry weight of the plant is mostly due to Carbon.

Fresh weight of the plant is mostly due to Oxygen.

2) Protoplasmic elements: C, H, O, N, P, S are considered as protoplasmic elements.
After C, H and O, the most abundant element present in the plant is N.

3) Energy related elements: Mg and P are the constituents of Chlorophyll and ATP respectively. Chlorophyll is concerned with conversion of radiant energy to chemical energy. P is useful in the intra cellular transportation of energy.
4) Some elements act as activators of certain enzymes. They are not functional in the absence of these activations.

Some of the examples are
         Mo → Dinitrogenase, Nitrate reductase
         Cu → Cytochromic oxidase, Phenolase, Ascorbic acid oxidase
         Zn → Carboxylase, Carbonic anhydrase.
         Mn → Malic dehydrogenase, IAA oxidase, Nitrite reductase.
         Fe → Catalase, Peroxidase
         Mg → Hexokinase, Phosphokinase
         Ca → NAD oxidoreductase, NAD kinase
5) Potassium alters the osmotic potential of the cell and thus helps in the movements opening of stomata.
6) Mobile elements: Old leaves supply certain elements to the young leaves through phloem. These are called mobile elements. Deficiency symptoms first appear in old leaves.
e.g.: N, P, K, Mg, Zn, Mo, Cl
7) Immobile elements: These elements are not supplied by the older leaves to the younger leaves as these are structural components.
e.g.: S, Ca, Fe, Cu, B
Mo is the element present in the least amount in plant body.

8) Non - mineral essential elements C, H and O are non - mineral in nature. N is considered as both mineral and non - mineral element.
9) Mineral elements: P, S, Mg, Ca, K, Fe, Cu, Mo, Mn, Zn, Cl, Ni are considered as mineral essential elements.

Deficiency symptoms
            If the elements are formed less than critical concentration the plants show deficiency symptoms.
Critical concentration
            The concentration of the essential elements below which plant growth in retarded is called critical concentration.
Common symptoms
1. Chlorosis: Loss of chlorophyll is called chlorosis. Leaf looks yellow.
Deficiency N, K, Mg, S, Fe, Mn, Zn, Mo.
Chlorosis first in older leaves is due to the deficiency of K, N, Zn.
Chlorosis first in younger leaves is due to the deficiency of S.
Mottled chlorosis is due to the deficiency of K. Chlorosis along margin is due to the deficiency of Ca & K.
2. Necrosis: Death of the chlorotic tissue is called Necrosis. It is due to the deficiency of Ca, Mg, Cu, K.
3. Cell division: It is inhibited by the deficiency of N, K, S, Mo.

4. Delay in flowering: It is due to the deficiency of N, S, Mo.
5. Excess symptoms: Nitrogen if absorbed in excess vegetative growth will be more but with less number of flowers and fruits .

Radioactive isotopes which are used to trace out the metabolic

pathways are called tracer elements. e.g.: 14C, 15N, 18O etc.

Toxicity of Micro elements
Excess manganese inhibits the uptake of Mg and Fe. It competes with Mg while binding to enzymes. It also inhibits the transport of Ca to stem apex. Thus excess of Mn causes.

Nitrogen Cycle
              The uptake of atmospheric nitrogen from the soil by the plants with the help of microorganisms, their utilisation and the return of N₂ to the atmosphere from soil is called Nitrogen Cycle or The Cyclic movement of Nitrogen from atmosphere to the soil and from soil back to the atmosphere through plants, animals & microorganisms is called as nitrogen cycle.
It is all Nitrogen
              It constitutes 78% of the atmosphere. It is molecular (N ≡ N), inert and also called trinitrogen. It can not be absorbed by the plants. Only certain prokaryotes which have dinitrogenase enzyme are capable of absorbing that form of nitrogen. Higher plants absorb nitrogen in the form of NO₃^- , NH₄⁺, NH₃. But NO₃^- is the premier form of nitrogen absorbed by plants. Nitrogen is the most important and abundant element found in the plant after C, H and O.
Nitrogen cycle has steps
              1. Nitrogen fixation                                       4. Nitrification
              2. Nitrogen assimilation                               5. Denitrification.
              3. Ammonification

1. What is Nitrogen fixation?
Conversion of atmospheric molecular nitrogen to NH₃ is called Nitrogen fixation or Diazotrophy.
              There are 2 kinds of Nitrogen fixation.
1. Abiological Nitrogen fixation.

2. Biological Nitrogen fixation.
Abiological Nitrogen fixation:
It occurs without the involvement of microorganisms. Thunders and sparks in the sky are responsible for it. Then it occurs as follows.
              N₂ + O₂ → 2 NO (Nitric Oxide)
              2 NO + O₂ → 2 NO₂ (Nitrogen dioxide)
              2 NO₂ + Rain H₂O → HNO₂ (Nitrous acid)/ HNO₃ (Nitric acid)
              HNO₃ after reaching the soil unites with Ca, K etc to form salts.
              HNO₃ + Ca → Calcium Nitrate
              HNO₃ + K → Potassium Nitrate

             These salts are soluble in water. They are absorbed by plants. Ammonia is produced in the laboratory by Haber - Bosch process at 450 °C and 100 bars of pressure.
Biological Nitrogen fixation (Diazotrophy): It occurs with the help of micro organisms called Diazotrophs or Nitrogen fixers. These are 2 types.
A. Asymbiotic: These microorganisms are free living, autotrophic and fix nitrogen to the plants without taking any help. They include
1. Bacteria
Aerobic, Chemoautotrophic - Azotobacter
Anaerobic, Chemoautotrophic - Clostridium
Anaerobic, Photosynthetic - Chlorobium, Rhodospirillum
2. Blue green Algae: e.g.: Nostoc, Anabaena
These plants have heterocysts which have nif genes.
B. Symbiotic
1. Blue green algae also fix nitrogen by symbiotic method.
2. Bacteria

       Rhizobium fixes nitrogen in the root nodules of leguminous plants and also non leguminaceae plants like Parasponia. Frankia fixes nitrogen in the root nodules of non leguminaceae plants like Alnus, Casuarina & Myrica.
Nitrogen Assimilation:
      Conversion of Inorganic nitrogen to organic nitrogen in the plants is called Nitrogen assimilation. It is an endergonic process. It occurs without the help of microbes. NH3 is used to produce Aminoacids, proteins, enzymes, DNA, RNA, ATP, Vitamins etc. Assimilatory products are also utilised by animals.
Ammonification:
      Dead and decomposing plant and animal parts are converted to ammonia in the soil by saprophytic micro organisms is called Ammonification. Most of it is absorbed by plants and the rest escapes into atmosphere.
Nitrification: Ammonia remaining in the soil is converted NO3 as follows.
             2 NH₃ + 3 O₂ → 2 NO₂^- + 2 H⁺ + 2 H₂O ............. (1)
             2 NO₂^-  + O₂ → 2 NO₃^- .............. (2)

       In the first step Nitrosomonas/ Nitrococcus participate. In the 2nd step Nitrobacter participates. These bacteria are obligatory, aerobic, chemoautotrophic. It is an exergonic process.
Denitrification
             NO₃^- is reduced to N₂ by denitrifying bacteria.
e.g.: Theobacillus denitrificans, Pseudomonas, Micrococcus. These bacteria are obligatory anaerobic free living chemoautrophic. It is an exergonic process. This N2again escapes into atmosphere.

Mechanism of Biological Nitrogen fixation

        8 H⁺, 8 e^- and 16 ATP are together called reducing power. These are supplied by
Pyruvic acid formed in glycolysis during respiration.
Dinitrogenase: It is a conjugate, metalloenzyme. It has 2 parts. 1. Tetramer 2. Dimer. Tetramer has Iron & Molybdenum. Dimer has Iron only. The enzyme requires anaerobic conditions as it is sensitive to oxygen. It becomes denatured non functional in the preserve of oxygen. This enzyme is produced only in the specific micro organism which have Nif genes.
            Nitrogenase enzyme takes first 8 e^- through Ferredoxin with the consumption of 16 ATP. 8 H⁺ are taken by the enzyme directly. These are added to N₂ to produce NH₃.
Nodule formation: It can be described step wise as follows.
1. The roots of leguminaceae plants secrete Sugars, Amino acids and flavonoids to attract bacteria.
2. The roots of the host select Rhizobium by secreting Lectin proteins.

3. Rhizobia secrete curling factor due to which root hairs show Shepherds Crook (a hook like structure).
4. Rhizobia produce cellulase. Cell wall of root hairs dissolve.
5. Rhizobia reach the cell wall & push plasma membrane which invaginates upto cortical cells to form infection thread.
6. Rhizobia reach the cortical cells. They become excited and produce auxins. Cortical cells divide and divide to produce swelling called nodule.
7. Bacterial cells which were rod shaped now become spherical and called as Bacteroids.
8. The Enzyme in Bacteroids becomes functional. The nodule produce a pink coloured pigment called Leghaemoglobin.
9. Nitrogenase fixes nitrogen to the cells of root nodule. It is protected by leghaemoglobin from oxygen.
10. Root nodules develop direct connection with vascular bundles of the root.

Leguminaceae plants grown in tissue culture. Laberatory donot develop root nodules, deficiency of Boron results in decreased modulation in the leguminaceae

                                                

Q. Differences between Rhizobium and Bacteroid.

Fate of Ammonia:
Ammonia is protonated. NH₄⁺ is formed.
               NH₃ + H⁺  NH₄⁺ (Reduction)
               NH₄⁺ is toxic. It can not be accumulated.
NH₄ + α − Ketoglutaric acid (Krebs cycle Intermediate)

         
Glutamic acid is the first amino acid produced in the plants. It is called Reductive amination or Amino acid Biosynthesis.

Transamination: Transfer of amino group from Glutamic acid to other keto acid is known as transamination.

       
           The amino acids (Glutamic acid, Aspartic acid) unite with amino group and amides are formed.

           
These amides transported to other parts through xylem.